1. Field of the Invention
This invention relates to a fluorescent lamp comprising a pair of electrodes and, more specifically, to a fluorescent lamp in which one of the electrodes is provided outside a discharge chamber as an external electrode and the other electrode is provided inside the discharge chamber as an internal electrode so as to cause discharge through a tubular glass bulb which is a dielectric.
2. Background Art
FIG. 4 shows an example of this type of fluorescent lamp 90 of the prior art which comprises a tubular glass bulb 91 having a fluorescent layer 92 formed on the inner surface and a pair of electrodes. The tubular glass bulb is sealed at both ends, air is exhausted from and a gas is charged into the tubular glass bulb 91 to form a discharge chamber 91a. One of the electrode is an external electrode 93 provided on the outer surface of the tubular glass bulb 91.
The other of the electrodes is an internal electrode 94 formed of a metal wire and provided substantially at the center in an axial direction of the tubular glass bulb 91 in the discharge chamber 91a. To prevent excessive tensile stress or sag generated by the difference of thermal expansion coefficient between the tubular glass bulb 91 and the metal wire (i.e., the internal electrode 94), the metal wire is formed into a coil and is given appropriate tension when it is installed.
In the fluorescent lamp 90 of the prior art described above, since a coil is used as the internal electrode 94, the problem caused by the difference of thermal expansion coefficient is solved. However, the internal electrode 94 resonates with vibration, freely vibrates and contacts the fluorescent layer 92 formed on the inner surface of the tubular glass bulb 91, thereby scratching or removing the fluorescent layer 92 from the glass bulb 91.
This problem cannot be ignored because the fluorescent lamp 90 may be used as a back light source for a liquid crystal display which is used for a car TV receiver or a car navigation system in many cases and is easily vibrated by the running of a vehicle.